"...since I recall a convo on these boards about a game (using MMC1) which relied on this behavior. I wish I could remember the name of the game and/or find the thread though."
The game was "Bill & Ted's excellent video adventure" or something like that. And it didn't rely on this in-between-write-back, it got screwed up because of it. There are other games just like it, if I was at home right now I could give you a list... :)
FCEu had a "too fast write" hack that prevented the MMC1 from getting screwed up on these RMW reg writes, whereas some other guy used another hack where he resetted the bit counter every time a new MMC1 reg was written to (works for all games, since it would be good programming practice to write 5 consecutive bits to the same reg)
Anyway, this writing back of unmodified values bugs me too, but it seems like that's the way it works... But I still can't figure out how the MMC1 doesn't get screwed on a real NES.